This invention relates to the cloth feed mechanism disposed across the cloth advancement path in a sewing machine and more particularly, to an improved upper layer feed dog arrangement for the cloth feed mechanism disposed above and facing the cloth feed mechanism to feed the upper layer of a cloth to be sewn in cooperation with the underlying cloth feed mechanism.
One prior art upper layer feed dog arrangement of the above type is disclosed in Laid-Open Japanese Utility Model Application No. 72451/1975. However, since the prior art upper layer feed dog arrangement comprises an upper layer feed dog supported on an upper layer feed bar which is connected at the rear end to an under layer feed bar on which an under layer feed dog is supported, the upper and lower layer feed dogs which grip a cloth to be sewn therebetween have the same cloth feed amount.
However, in the sewing machine incorporating such an upper layer feed dog arrangement therein, since the cloth feed machanism comprising the pressure foot disposed on one side of the cloth advancement path in the sewing machine and the opposing feed dog disposed on the other side of the cloth advancement path in face-to-face relationship to the pressure foot is present in the sewing area and the feed amount of the upper layer of the two superimposed cloth layers to be sewn together is usually less than that of the lower layer because the upper layer is subjected to frictional resistance offered by the undersurface of the pressure foot and relative slippage between the upper and lower layers, a relative displacement tends to occur between the upper and lower layers. And such relative displacement varies depending upon the coefficient of friction of a particular cloth to be sewn.
Therefore, when the two layers of a cloth disposed one upon another are sewn together in the prior art sewing machine, the relative displacement between the upper and lower layers can not be compensated for by the upper and lower layer feed dogs resulting in a low quality cloth product and the prior art upper layer feed dog arrangement can not accommodate different types of cloth having varying coefficients of friction.